C#: Cache IRType like for C++

csharp/ql/src/semmle/code/csharp/ir/implementation/IRType.qll

@@ -4,6 +4,7 @@
 
 private import internal.IRTypeInternal
 
+cached
 private newtype TIRType =
   TIRVoidType() or
   TIRUnknownType() or

csharp/ql/src/semmle/code/csharp/ir/internal/CSharpType.qll

@@ -144,6 +144,7 @@ private IRType getIRTypeForPRValue(Type type) {
 
 string getOpaqueTagIdentityString(Type tag) { result = tag.getQualifiedName() }
 
+cached
 private newtype TCSharpType =
   TPRValueType(Type type) { exists(getIRTypeForPRValue(type)) } or
   TGLValueAddressType(Type type) { any() } or
@@ -163,6 +164,7 @@ class CSharpType extends TCSharpType {
    * Gets the `IRType` that represents this `CSharpType`. Many different `CSharpType`s can map to a
    * single `IRType`.
    */
+  cached
   abstract IRType getIRType();
 
   /**